1. Field of the Invention
The present invention relates to apparatus and methods for economically forming precision molded articles from mixes of refractory particles and heat fugitive organic binders.
2. Description of the Prior Art
Although it will be evident that the present invention has broad utility and can be used to form many kinds of molded refractory articles, the invention and its background will be described principally in the context of the fabrication of inserts for plastic injection molds.
Techniques for molding sintered metal articles from mixes of refractory particles or powders and heat fugitive organic binders have long been known. Such techniques can be used, for example, to make sintered metal core and cavity inserts for durable injection molds capable of producing millions of plastic injection molded parts. In a typical process currently in use, a flexible molding material such as RTV (room temperature vulcanized) silicone rubber is poured around a master pattern and allowed to cure. Once the RTV has cured, the master pattern is removed and a mixture of refractory metal particles (for example, A.sub.6 tool steel particles and tungsten carbide particles) and a heat fugitive organic binder (for example, epoxy liquid) is poured into the RTV mold and allowed to cure. The cured mixture, or green article, is removed from the RTV mold, heated to burn off the binder and tack the refractory particles, sintered and then infiltrated with a low melting point infiltrant such as copper. The result is a metal composite refractory article having both high hardness and high impact strength making the technique useful for producing tooling such as a plastic injection mold insert, referred to herein as an "infiltrated powdered metal composite tooling insert" or "IPMCTI". Production and use of tooling formed using powder metal technology are known in the art. Examples of techniques for such production and use are disclosed in the following U.S. Patents that are each herein incorporated by reference in their entirety:
(1) U.S. Pat. No. 3,823,002, entitled "Precision Molded Refractory Articles," issued July 1974 to Kirby et al. PA1 (2) U.S. Pat. No. 3,929,476, entitled "Precision Molded Refractory Articles and Method of Making," issued December 1975 to Kirby et al. PA1 (3) U.S. Pat. No. 4,327,156, entitled "Infiltrated Powdered Metal Composites Article," issued April 1982 to Dillon et al. PA1 (4) U.S. Pat. No. 4,373,127, entitled "EDM Electrodes," issued February 1983 to Hasket et al. PA1 (5) U.S. Pat. No. 4,432,449, entitled "Infiltrated Molded Articles of Spherical Non-Refractory Metal Powders," issued February 1984 to Dillon et al. PA1 (6) U.S. Pat. No. 4,455,354, entitled "Dimensionally-Controlled Cobalt Containing Precision Molded Metal Article," issued June 1984 to Dillon et al. PA1 (7) U.S. Pat. No. 4,469,654, entitled "EDM Electrodes," issued September 1984 to Hasket et al. PA1 (8) U.S. Pat. No. 4,491,558, entitled "Austenitic Manganese Steel Containing Composite Article," issued January 1985, to Gardner. PA1 (9) U.S. Pat. No. 4,554,218, entitled "Infiltrated Powdered Metal Composite Article," issued November 1985, to Gardener et al. PA1 (10) U.S. Pat. No. 5,507,336, entitled "Method of Constructing Fully Dense Metal Molds and Parts," issued to Tobin. PA1 1. Reduced cost: Currently, the entire volume of a molded refractory article is generated using relatively expensive refractory mix. Employing the present invention, a significant fraction of the volume of the article is fabricated using substantially lower cost conventional tool steel refractory mix displacement elements which may take various forms but which preferably comprise rods. In the case of an IPMCTI, the smaller volume of the substantially more expensive refractory mix is employed mainly to generate the critical geometry of the active tooling surface, as well as the interstitial volume between the mix displacement elements. PA1 2. Improved accuracy: Since the total volume of refractory mix undergoing shrinkage is substantially reduced, the overall process shrinkage is also reduced, leading to improved accuracy of the green and final articles. PA1 3. Enhanced process yield: At present, a significant fraction of all molded refractory articles are destroyed due to breakage in the green state. Since tool steel is much stronger than a green refractory article, breakage is reduced, resulting in improved process yield. PA1 4. Reduced bowing distortion: The increased strength of the steel rods serves to reduce distortion of the green refractory article.
Although the described technique is relatively straightforward, it has several shortcomings in the fabrication of molds, tooling, tooling inserts, usable articles, or full size prototype devices:
First, the effective cost per cubic mm of an IPMCTI is many times greater than that of an insert made from conventional tool steel. For especially small inserts (for example, 25-75 cu. mm) this difference in material cost is not significant relative to set-up charges or incremental charges associated with increased speed of delivery. However, for larger inserts (for example, 250 cu. mm and greater), the material cost penalty for the entire tool (core and cavity) becomes substantial and the process becomes far less economically attractive. In particular, the mixture of refractory particles and binder can be a significant part of the costs to prepare a final part.
Second, the green article shrinks appreciably during sintering and infiltrating. Such shrinkage may be nonuniform when sections of significantly different thicknesses exist in an article, and especially when such sections share a common intersection; nonuniform shrinkage often results in part distortion and decreased precision of the article.
Third, the green article is relatively fragile and is therefore subject to breakage during demolding with consequent decreases in process yield. This is especially a problem with larger parts. The articles in their final, sintered state are also subject to breakage.
Accordingly, it is an object of the present invention to reduce the cost per unit volume of molded refractory articles.
It is another object of the present invention to reduce shrinkage and especially nonuniform shrinkage and the resulting distortion and decreased precision of molded refractory articles.
It is still a further object of the present invention to increase the strength of molded refractory articles both in the green state, sintered state and in the optionally final, infiltrated state.
It is a further object of the invention to pursue the above and other objects, which will be apparent from the instant disclosure, alone and in various combinations.